The DataReader is responsible for reading typed sample and sample information from the middleware and is equivalent to the DataWriter, the DataReader and DataWriter provide the bulk of Listeners, conditions and QoS available in DDS as they're the intermediaries between the middleware and the application.

The DataReader is an abstract class, that is always sub-classed by a typed specialisation, which is generated by the pre-processor:

For a type Foo the preprocessor generates a FooDataReader
- This typed subclass contains all the functions or methods that pass parameters of type Foo
  - This was, type-safety is ensured by the compiler
- Since the Subscriber is type-independent, it can not return a typed DataReader: the result will have to be down-casted to its proper type by the application

Applicable QoS

QoS	Brief
USER_DATA	User data
DURABILITY	Expresses the lifetime of a sample
DEADLINE	Sets the period in which a sample must be received
LATENCY_BUDGET	Specifies the maximum acceptable delay for a sample in transit
LIVELINESS	Set the method by which an instance is considered alive (publishing)
OWNERSHIP	Specifies if samples should be treated as having a single of multiple owners
TIME_BASED_FILTER	Filter based on the time separation between samples
RELIABILITY	Set the required reliability of a DataReader/DataWriter pair, reliable or best effort
DESTINATION_ORDER	Controls the logical order of the samples, by reception time-stamp or source time-stamp
HISTORY	Specify how many generations of the same instance to keep
RESOURCE_LIMITS	Specify the amount of resources the DataReader can consume
READER_DATA_LIFECYCLE	Specify the lifecycle for data-instances
SUBSCRIPTION_KEYS	Allows the DataReader to define its own set of keys on the data, different from the keys defined by the topic
READER_LIFESPAN	Automatically remove samples from the DataReader after a specified timeout.
SHARE	Used to share a DataReader between multiple processes

Default QoS values

QoS	Attribute	Value
USER_DATA	value.length	0
DURABILITY	kind	VOLATILE
DEADLINE	period	DURATION_INFINITE
LATENCY_BUDGET	duration	0
LIVELINESS	kind lease_duration	AUTOMATIC DURATION_INFINITE
OWNERSHIP	kind	SHARED
TIME_BASED_FILTER	minimum_separation	0
RELIABILITY	kind max_blocking_time synchronous	BEST_EFFORT 100 ms FALSE
DESTINATION_ORDER	kind	BY_RECEPTION_TIMESTAMP
HISTORY	kind depth	KEEP_LAST 1
RESOURCE_LIMITS	max_samples max_instances max_samples_per_instance	LENGTH_UNLIMITED LENGTH_UNLIMITED LENGTH_UNLIMITED
READER_DATA_LIFECYCLE	autopurge_nowriter_samples_delay autopurge_disposed_samples_delay autopurge_dispose_all enable_invalid_samples invalid_sample_visibility.kind	DURATION_INFINITE DURATION_INFINITE FALSE TRUE MINIMUM_INVALID_SAMPLES
SUBSCRIPTION_KEYS	use_key_list key_list.length	FALSE 0
READER_LIFESPAN	use_lifespan duration	FALSE DURATION_INFINITE
SHARE	name enable	NULL FALSE

Related Status Conditions

Status	Brief
SAMPLE_REJECTED	A received sample has been rejected
LIVELINESS_CHANGED	The liveliness of a DataWriter has changed
REQUESTED_DEADLINE_MISSED	A deadline was not met
REQUESTED_INCOMPATIBLE_QOS	A QoS policy was not compatible with the offering
DATA_AVAILABLE	New information available
SAMPLE_LOST	A sample has been lost
SUBSCRIPTION_MATCHED	Found matching DataWriter with compatible QoS and Topic

Related Listeners

Listener	Brief
on_sample_rejected()	A received sample has been rejected
on_liveliness_changed()	The liveliness of a DataWriter has changed
on_requested_deadline_missed()	A deadline was not met
on_requested_incompatible_qos()	A QoS policy was not compatible with the offering
on_data_available()	New information available
on_sample_lost()	A sample has been lost
on_subscription_match()	Found matching DataWriter with compatible QoS and Topic